1. Field of the Invention
The present invention is directed to voltage-controlled oscillator, and more particularly to a gain-adjustable voltage controlled oscillator used in Voltage control circuit design for the controlled oscillation and output of a desired frequency.
2. Description of the Prior Art
In conventional Phase-Locked Loop (PLL) circuits, a voltage controlled oscillator (VCO) is used for generating an oscillation. The parameters of the VCO, such as the frequency gain (Ko) and the frequency range directly affect the loop characteristics and the output frequency range of the PLL.
The VCO has traditionally been designed in a number of different ways. The most popular design is the ring oscillator, which consists of an odd number of inverting stages connected in a ring. A state voltage controls the delay of each stage, and the VCO toggling frequency is controlled accordingly. The state voltage is the result of the PLL feedback comparison. The PLL circuit compares the VCO frequency (or a fraction thereof) against the reference clock, making adjustments to the state voltage dynamically to tune the VCO frequency to the desired value.
The frequency gain (Ko) of a VCO is the ratio of the increment of the VCO angular frequency by the state voltage adjustment ((.delta..omega.)/(.delta.V), in rad/s/NV). As Ko increases, VCO becomes more sensible to the state voltage change. In designing PLL circuits, the frequency gain Ko is one of the most important parameters to determine the PLL characteristics and must be designed within the proper range.
However, the current use of a distributed fabrication process for the design and manufacture of PLL circuits and the use of PLL circuits in diverse application situations (e.g. 5V/3V choices, etc.) typically results in a frequency gain which may deviate severely from the expected range. Accordingly, a voltage control circuit is needed which is adjustable after fabrication so that it could be tuned to the proper range based upon field measurements.